He's 'ok' because of his high HP stat but there's better clerics for the job. Sylveon hits harder than Vaporeon and is better at taking Special attacks. Clefable is a tankier cleric and is great against setup sweepers if you run her with Unaware. If your team is ok with having a very passive 'mon then you can take Chansey or Blissey (Chansey is better).Is cleric Vaporeon (wish/healbell/sub/baton pass maybe? kinda weird without any offense, maybe I should drop something for scald) is a decent idea of support for Mega Metagross?
if not, what other cleric would you recommend?
Edit: It should also be noted that Clefable and Chansey have access to immediate recovery in Soft-boiled for Clefable with Magic Guard and Chansey and Moonlight for Unaware Clefable. Wish/Protect is also viable but you should take the immediate recovery into account.